Continuing use of digital cameras including still and video cameras on portable devices has created a demand for image sensors. The use of cameras on portable and battery powered devices such as tablet computers, smartphones, laptops and web-based video players continues to increase.
Early digital cameras primarily relied on charge coupled devices (“CCDs”) as image sensors. More recently CMOS image sensors (“CIS”) have become increasingly prevalent. CCIS technology provides a digital device, as opposed to an analog device, and because the CIS uses CMOS transistor or diode technology, additional digital processing and logic circuitry may easily be incorporated into an imaging integrated circuit. The efficiency of light gathering (quantum efficiency or “QE”) may be greater than that achieved with a CCD device.
A CIS device is based on a light sensitive photo-diode, which may be formed as a diode or as part of a transistor, a “photo-transistor”. When the photo-diode is exposed to light, electrons flow in proportion to the light, and a voltage signal may be produced that indicates the amount of light received in a given time period. This signal is then converted to a digital signal and circuitry on the device may output the signal. An array of picture elements (“pixels”) is formed on the semiconductor device. Additional circuitry may be formed outside of the array to provide digital signal processing such as filtering, and image processing, for example, and to provide interface circuitry to a system.
Originally front side illuminated (“FSI”) arrays were used with CIS devices. However the light impinging on a photo-sensor in a FSI CIS has to pass through layers of metal and dielectric that may absorb or scatter the impinging light, thus reducing the quantum efficiency. The need for use of the devices in low light conditions results in a demand for a high QE. Back side illuminated (“BSI”) CIS devices have been developed. By receiving incoming light from the backside of the semiconductor substrate, the light path may be made short and without the intervening layers of metal and dielectric. By using wafer thinning and with the use of epitaxial semiconductor layers, the substrate may be further thinned or even removed and the photo-sensor is then very near the surface of the backside of the device, greatly increasing the QE. BSI CIS devices are increasingly prevalent in current products.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the preferred embodiments and are not necessarily drawn to scale.